In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications (GSM)/Enhanced Data rate for GSM Evolution (EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. User equipments are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data to the user equipments in downlink (DL) transmissions.
Multi-antenna techniques may significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The LTE standard is currently evolving with enhanced MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. For instance there is in LTE-Advanced support for a spatial multiplexing mode with possibly channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. An illustration of the spatial multiplexing operation is provided in FIG. 1. An information carrying symbol vector s is multiplied by an ‘NT×r’ precoder matrix WNT×r, which serves to distribute the transmit energy in a subspace of the NT (corresponding to NT antenna ports) dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices, and typically indicated by means of a precoder matrix indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. The precoded signals are then Inverse Fast Fourier Transformed (IFFT).
In this way, spatial multiplexing is achieved since multiple symbols may be transmitted simultaneously over the same time/frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
LTE uses Orthogonal frequency Division Multiplexing (OFDM) in the downlink, and Discrete Fourier Transform (DFT) precoded OFDM in the uplink, and hence the received NR×1 vector yn, where NR is number of receiver ports, for a certain TFRE on subcarrier n, or alternatively data TFRE number n, is thus modeled byyn=HnWNT×rSn+en where en is a noise/interference vector obtained as realizations of a random process. The precoder matrix, WNT×r may be a wideband precoder, which is constant over frequency, or frequency selective. Note that the signals above, e.g. yn, may alternatively represent a signal in a time-domain. It is generally understood that signals mentioned may represent signals in other domains than in the time-frequency grid of an OFDM system.
The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced.
In closed-loop precoding for the LTE downlink, the UE transmits, based on channel measurements in the forward link (downlink), recommendations to the radio base station of a suitable precoder to use. The UE selects a precoder out of a countable and finite set of precoder alternatives, referred to as a precoder codebook. A single precoder that is supposed to cover a large bandwidth, wideband precoding, may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g. several precoders, one per subband of the large bandwidth. This is an example of the more general case of Channel State Information (CSI) feedback, which also encompasses feeding back other entities than precoders to assist the radio base station in subsequent transmissions to the UE. Such other information may include Channel Quality Indicators (CQIs) as well as transmission Rank Indicator (RI).
For the LTE uplink, the use of closed-loop precoding means the radio base station is selecting precoder(s) and transmission rank and thereafter signals the selected precoder that the UE is supposed to use.
Already Release-8, the first release, of LTE supports codebook based precoding for 2 antennas, a so called 2 Tx Codebook. Up to two layers may be transmitted, rank 1 and rank 2, thus making the precoder matrix W2×r of dimension 2×1 and 2×2, respectively. The precoder 2 Tx Codebook comprises a total of six precoders
      W          2      ⁢                          ⁢      xr        ∈      {                  [                                            1                                                          1                                      ]            ,              [                                            1                                                                          -                1                                                    ]            ,              [                                            1                                                          j                                      ]            ,              [                                            1                                                                          -                j                                                    ]            ,              [                                            1                                      1                                                          1                                                      -                1                                                    ]            ,              [                                            1                                      1                                                          j                                                      -                j                                                    ]              }  out of which the first four precoders are seen to represent rank one and the rest rank 2.
LTE Release-10 and later supports a transmission mode for up to 8-layer spatial multiplexing for 8 Tx antenna ports using UE specific Reference Signal (RS), also referred to as a 8 Tx precoder codebook. An antenna port may not necessary correspond to a physical antenna but may also correspond to multiple antennas. Rank adaptation and possibly channel dependent precoding is also supported. UE specific RS is used for demodulation purposes and because of that the radio base station is free to use whatever precoder(s) it wants to, but it may be assisted in the determination of precoder(s) via CSI feedback from the UE that includes recommended precoder(s). For the time-frequency resource of interest, the UE selects a precoder out of a set of possible precoders in a precoder codebook. The available precoders in the precoder codebook are of a special factorized structure; a precoder may be written as a product of two matrix factors
                              W                      8            ×            r                          =                ⁢                              W                          8              ×              2              ⁢                                                          ⁢              k                                      (              c              )                                ⁢                      W                          2              ⁢                                                          ⁢              k              ×              r                                      (              t              )                                                              =                ⁢                              [                                                                                                      W                      ~                                                              4                      ×                      k                                                              (                      c                      )                                                                                        0                                                                              0                                                                                            W                      ~                                                              4                      ×                      k                                                              (                      c                      )                                                                                            ]                    ⁢                      W                          2              ⁢                                                          ⁢              k              ×              r                                      (              t              )                                          where an 8×2k conversion precoder W8×2k(c) strives for capturing wideband/long-term properties of the channel such as correlation while a 2k×r tuning precoder W2k×r(t) targets frequency-selective/short-term properties of the channel. Together they form the overall precoder Wg×r which together with an input symbol vector sr×1 produces an output signal x8×1=W8×rsr×1 for r layers. The parameter k is in LTE taken to be equal to four for rank 1 and 2. Further details concerning the LTE codebook are found in 3GPP TS 36.213 V10.4.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures” section 7.2 and 3GPP TS 36.211 v10.3.0 section 6.3.4.2.3.
In order for the UE to generate feedback regarding the current channel conditions a set of pre-defined channel state information reference signals (CSI-RS) may be transmitted from the radio base station to the user equipment. Based on the CSI-RS, the UE can estimate the channel and consequently also figure out which precoder suits the particular channel. For the purpose of CSI feedback determination, the UE assumes that each of the rows in x8×1 corresponds to an antenna port, ports 15-22, on which a CSI-RS is transmitted. The first row represents antenna port 15, second row antenna port 16 and so on. Each CSI-RS is typically transmitted from an antenna of its own, meaning that there is a direct correspondence between an antenna port and a physical antenna.
The design target of the 8 antenna-port LTE codebook was an 8 Tx antenna array with either four closely spaced cross-poles or eight closely spaced co-poles placed in a uniform and linear fashion. For the former case the first four rows of W8×2k(c) will target a first polarization (they are all co-polarized) and the remaining four antennas target a polarization orthogonal to the former polarization. Due to the structure of W8×2k(c), beamforming is conducted separately for each polarization followed by precoding between polarizations. For the case of eight co-poles, all the eight rows of matrix W8×2k(c) will be used to perform beamforming in one polarization. Beamforming is achieved by controlling phase and relative amplitude of the signal at each active transmitting antennas by combining elements in an antenna array in a way where signals at particular angles experience constructive interference and while others experience destructive interference.
Antenna Arrays
On the network side, radio base stations are often equipped with multiple antennas to be used for reception and transmission. The antennas intended for a cell, and/or a sector, form a so-called antenna array. Some typical antenna array constellations are illustrated in FIGS. 2(a)-(b). For instance, one common antenna array layout is to use co-polarized antennas in order to construct antenna arrays as shown in FIG. 2(a). Furthermore, another common layout is to instead use cross-polarized antennas as shown in FIG. 2(b). FIG. 2(a) shows 1 Tx, 2 Tx and 4 Tx co-polarized antenna arrays and FIG. 2(b) shows 2 Tx, 4 Tx and 8 Tx cross-polarized antenna arrays. Using for instance a 2 Tx cross-polarized antenna array, e.g. the top most antenna setup in FIG. 2(b), implies that the antenna array is fed with two signals, x1 and x2.
This is illustrated in FIG. 3 where it has been assumed that a 2 Tx antenna array is used with codebook based precoding and thus x2×1=W2×rsr×1. An example of a codebook W2×r was presented above. Thus, FIG. 3 shows an illustration of codebook based precoding based with a 2 Tx cross-polarized antenna array.
Active Antennas or Active Transmitting Antenna
An active antenna array comprises a number of sub elements or small physical devices that jointly form the active transmitting antenna. In FIG. 4(a) a sub element, in practice realized by a small physical device, is illustrated. Each sub element will have a polarization direction which potentially may be orthogonal to another sub element's polarization. This is illustrated in FIG. 4(b) where a sub element with orthogonal polarization compared to the sub element in FIG. 4(a) is shown. Finally, in FIG. 4(c) an active antenna array which comprises NC sub elements is shown. In general, but not necessarily, all the sub elements of an active transmitting antenna of an active antenna array are of the same polarization. Note that each given sub element j can be fed the given signal x(j) not necessarily equal to x(i), which is a signal for the active transmitting antenna i. Thus, FIG. 4(a) shows a sub element; FIG. 4(b) shows a sub element in the polarization orthogonal to the polarization of the sub element in FIG. 4(a); and FIG. 4(c) shows sub elements 1 . . . NC of an active antenna array comprising a active transmitting antenna i=1.
In this document, when dealing with more than one active transmitting antenna, we will adopt the notation xi(j) when referring to a signal, or function, related to the j:th sub element in the i:th active transmitting antenna. These indexes will however be omitted when it is clear from the context what is being referred.
By combining two active antennas of different polarizations, as illustrated in FIG. 5(a), a 2 Tx antenna array may be created and fed with two different signals, x1 and x2 where xi=[xi(1) . . . xi(Nc)]T, where i is the active transmitting antenna, and NC is the sub elements of the active transmitting antenna. Furthermore, by combining multiple 2 Tx antenna arrays, as illustrated in FIG. 5(b), an 8 Tx antenna array can be created. Here the signals xj2(j1) are no longer explicitly shown but they are still assumed to be present in the same manner as in FIG. 5(a). FIG. 5(a) thus depicts a 2 Tx active antenna array and FIG. 5(b) depicts an 8 Tx active antenna array.
Existing precoder codebooks in different standards have been designed for conventional antenna arrays. In for instance LTE Release 10 and beyond, precoder codebooks for 2, 4 or 8 Tx antenna ports are supported. There is thus a precoder codebook suitable for each antenna array type. Hence, when using for instance a 2 Tx antenna array the standard supports the use of the 2 Tx codebook meaning that x1 and x2 can be fed to the antenna array just as in FIG. 3.
An active antenna array comprises many sub elements and arrays of active antennas comprise even more. Such antenna setups were neither thought of, nor taken into account, when the existing codebooks were designed. Therefore, existing precoder codebooks do not utilize the fact that the sub elements can be accessed and fed with a signal as illustrated in FIG. 5. Today, there exists no manner to use an active antenna array of a number of active transmitting antennas for transmitting data signals in an efficient and reliable manner.